This invention relates to a fluidized bed combustor and, more particularly, to an improved particulate coal feed system for such a combustor.
The use of fluidized beds has long been recognized as an attractive way of generating heat. In a normal fluidized bed arrangment, air is passed through a bed of particulate material, which usually includes a mixture of inert material, and a fuel material such as high sulfur bituminous coal. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. The basic advantages of such an arrangement include a relatively high heat transfer rate, substantially uniform bed temperature, combustion at relatively low temperatures, a reduction in corrosion and boiler fouling and a reduction in boiler size.
In these arrangements, the particulate fuel material must be continuously, or at least periodically, distributed into the bed to replenish the spent material expended in the combustion process. Many conventional systems utilize a feeder, or feeders, for distributing the particulate fuel material from a position above the upper surface of the bed where it falls by gravity onto the latter surface. However, a great majority of the commercial coal that is available is of a relatively large particle size range, which may extend from very small particles to particles that will pass through a two-inch screen. Therefore, the distribution of the particulate fuel material onto the upper surface of the bed presents problems since, if the particulate fuel material is relatively small, the combination of the upwardly rising combustion gases and air passing through the bed will blow the material out of the top of the combustor before it is burned.
It also has been suggested to provide in-bed feeding systems in which the particulate fuel material is introduced directly into the bed from a point below the upper surface of the bed. Although these systems avoid the above problems, they present additional problems since the lateral transfer or dispersion of the materials through the bed is relatively poor, requiring a multiplicity of feed points to prevent hot spots or over-cool spots. This, of course, increases the cost of the installation.